This invention pertains to bidentate square planar complexes of triazolates and particularly to their uses in organic light emitting diodes (“OLEDs”), n-type semiconductor materials, and other applications. This invention also pertains to improved efficacy and/or color stability at high brightness in single- or two-emitter white or monochrome OLEDs that utilize homoleptic square planar complexes of the general type [M(N^N)2], wherein two identical N^N bidentate anionic ligands are coordinated to the M(II) metal center, as exemplified by the preferred embodiment bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II) (“Pt(ptp)2”).
Luminescent organic or metal-organic molecular materials have a range of applications. These include organic light emitting diodes (“OLEDs”) that exhibit white or monochrome electroluminescence. Such devices may be utilized in solid-state lighting (“SSL”), which can greatly decrease the energy demand of current lighting technologies that account for 22% of total electrical power consumption in the U.S., and also for video display in electronic devices such as TV, camcorders, monitors, cell phones, etc. In particular, utilization of phosphorescent metal-organic complexes in OLEDs has allowed higher device performance than that allowed by fluorescent organic materials because the phosphorescent metal-organic complexes allow radiative recombination of both triplet and singlet excitons (with an upper limit of 100% efficiency compared to 25% for fluorescent organic materials).
OLEDs have emerged as promising candidates for solid-state lighting and display applications. For monochromatic electroluminescent devices, 100% internal quantum efficiency has been reported using phosphorescent molecules that harvest both singlet and triplet excitons, representing a four-fold increase in efficiency compared to electrofluorescent devices. White OLEDs (“WOLEDs”) can now exceed the ˜12-17 μm/W power efficiency of conventional incandescent lamps. Efficient WOLED strategies include red, green, and blue phosphorescent dopants in a single emissive layer (D'Andrade, 2004), phosphor-doped host layers in a stacked configuration (Qi, 2008), and a stacked combination of fluorescent and phosphorescent dopants (Kanno, 2006). The most common approach for creating WOLEDs is typically the combination of multiple emitters, for example red, green, and blue, or RGB emitters. This approach usually requires sophisticated device structures and results in difficult-to-control processing conditions, differential aging of RGB emitters, and/or energy transfer to the red color center (Misra, 2006; D'Andrade, 2004). The first instance whereby a single dopant was used to produce white electrophosphorescence in an OLED with standard device structures and materials (D'Andrade, 2002) produced performance metrics that do not meet today's standards for SSL in power efficiency and device stability (D'Andrade, 2004). To replace conventional incandescent or fluorescent light sources, WOLEDs must exhibit high color rendering index (“CRI”), stability at a high luminance of ≧1000 cd/m2, and long operational lifetimes exceeding 10,000 hours.
Recently, higher external quantum efficiency and power efficiency were demonstrated in fluorescent/phosphorescent WOLEDs employing a blue fluorophore combined with green and red phosphors in a common host (Sun, 2006). The improvement in performance is attributed to judicious harvesting of singlet and triplet excitons. Resonant energy transfer from the host to dopants eliminates exchange energy loss, resulting in improved efficiencies.
Self-quenching and long radiative lifetimes in phosphorescent emitters are partly responsible for decreased efficiency and stability in OLEDs (Baldo, 1998; D'Andrade, 2004). Thus, new materials and device concepts that overcome these issues are critically needed for OLEDs with improved efficacy and/or color stability at high brightness.